[230]连接Redis后执行命令错误 MISCONF Redis is configured to save RDB snapshots

今天在redis中执行setrange name 1 chun 命令时报了如下错误提示：

(error) MISCONF Redis is configured to save RDB snapshots, but is currently not able to persist on disk. Commands that may modify the data set are disabled. Please check Redis logs for details about the error．

大意为：（错误）misconf redis被配置以保存数据库快照，但misconf redis目前不能在硬盘上持久化。用来修改数据集合的命令不能用，请使用日志的错误详细信息。

这是由于强制停止redis快照，不能持久化引起的，运行info命令查看redis快照的状态，如下：

解决方案如下：

连接redis后运行　config set stop-writes-on-bgsave-error no　命令

关闭配置项stop-writes-on-bgsave-error解决该问题。

今天第二次遇到Redis “MISCONF Redis is configured to save RDB snapshots, but is currently not able to persist on disk”的问题。这个错误信息是Redis客户端工具在保存数据时候抛出的异常信息。

网上查了一下，很多人都是建议“config set stop-writes-on-bgsave-error no”。这样做其实是不好的，这仅仅是让程序忽略了这个异常，使得程序能够继续往下运行，但实际上数据还是会存储到硬盘失败！

上一次遇到这个问题是因为一个程序的Bug造成系统内存被耗尽了，后来修复了那个Bug问题就解决了。今天出现问题时查看系统内存还有2GB左右，“感觉好像不是内存的缘故”（后面发现还是因为内存的缘故）。

由于Redis是daemon模式运行的，没法看到详细的日志。修改配置文件设置logfile参数为文件（默认是stdout，建议以后安装完毕就修改这个参数为文件，不然会丢掉很多重要信息），重启Redis，查看日志，看到程序启动时就有一行警告提示：
“WARNING overcommit_memory is set to 0! Background save may fail under low memory condition. To fix this issue add ‘vm.overcommit_memory = 1’ to /etc/sysctl.conf and then reboot or run the command ‘sysctl vm.overcommit_memory=1’ for this to take effect.”（警告：过量使用内存设置为0！在低内存环境下，后台保存可能失败。为了修正这个问题，请在/etc/sysctl.conf 添加一项 ‘vm.overcommit_memory = 1’ ，然后重启（或者运行命令’sysctl vm.overcommit_memory=1’ ）使其生效。）

当时没明白意思，就忽略了。再启动Redis客户端，程序保存数据时继续报“MISCONF Redis is configured to save RDB snapshots, but is currently not able to persist on disk”异常，再查看Redis日志，看到有这样的错误提示“Can’t save in background: fork: Cannot allocate memory”，这个提示很明显"Fork进程时内存不够用了！"（还是内存的问题）。

通过谷歌查询“Can’t save in background: fork: Cannot allocate memory”这个提示，找到了解决方法：

// 原文：http://pydelion.com/2013/05/27/redis-cant-save-in-background-fork-cannot-allocate-memory/
If you get this error  Can't save in background: fork: Cannot allocate memory  it means that your current database is bigger than memory you have. To fix the issue enable vm.overcommit_memory:  sysctl vm.overcommit_memory=1  To have if after reboot add this line to /etc/sysctl.cnf:  vm.overcommit_memory=1

修改vm.overcommit_memory=1后问题果然解决了。

为什么系统明明还剩2GB的内存，Redis会说内存不够呢？

网上查了一下，有人也遇到类似的问题，并且给出了很好的分析（详见：http://www.linuxidc.com/Linux/2012-07/66079.htm），简单地说：Redis在保存数据到硬盘时为了避免主进程假死，需要Fork一份主进程，然后在Fork进程内完成数据保存到硬盘的操作，如果主进程使用了4GB的内存，Fork子进程的时候需要额外的4GB，此时内存就不够了，Fork失败，进而数据保存硬盘也失败了。

redis错误：LOADING Redis is loading the dataset in memory

原因是，redis使用的内存超过操作系统一半的内存

查看文件占的内存

du -sh ./*|grep G
df -h

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